High pitch Noise

what kind of boiler did you have that the hx was clogged? that is an unusual thing to happen to a cast iron hot water boiler, at least on the water side.

It is — when the pumps aren't running. As soon as a pump starts, it creates a pressure difference across itself; that's why the water moves. That may easily be 10 pounds, lower on the inlet side.

Pumping is a common way to control two zones. So are two zone valves. Both have their advantages — and disadvantages.

Correct. Now the trick is that the pressure at the expanson tank is held steady by the tank, so depending on where the pump is in relation to the tank, the pressure at the inlet may be less than the pressure at the expansion tank — perhaps quite a bit less. Now apparently the pumps are on the returns in your system, and the expansion tank isi on the supply. This means that the whole pressure loss of the system shows up as a low pressure on your pump inlets — and this can be problematic for a number of reasons.

Noise being one of them…

If you had a common return, yes — but as I understand it you have two pumps, one on each return from the loops? The tank has to be connected BEFORE the inet to the pump, and with two pumps in parallel…

Here is one idea on how to place the circulators to pump away from the expansion tank.

And here is some simple math to help understand the issue:

One Taco 007e circulator can move roughly 10 gallons per minute in many typical residential hydronic systems. Now you have two circulators, so in theory you could move up to about 20 gallons per minute.

Your 3/4-inch piping generally operates nicely at around 4 gallons per minute per loop. If you have two zones, the system would normally work well at about 8 gallons per minute total.

So you have a system capable of moving around 20 gallons per minute, connected to a distribution system that really only needs about 8 gallons per minute. That suggests that whoever installed the boiler may not have done the math before selecting two circulators to replace two zone valves that had worked fine for years.

A new zone valve typically costs a little over $100, while those circulators can cost over $225 each at wholesale. After retail markup, you may have paid over $300 more to install two circulators where only one circulator and two zone valves might have been sufficient.

Of course, all of that is buried in the total job cost, so the homeowner rarely sees it itemized. In addition, using zone valves would likely have required less piping modification around the boiler, which could have reduced the installation labor as well.

Some installers have a philosophy of never using zone valves, while others prefer them. In many cases, however, the choice may not involve much engineering thought. Sometimes the preference simply comes down to what the installer is most comfortable wiring or servicing, or which option appears easiest or cheapest at the time. Unfortunately, that approach does not always consider the mechanical design of the system and which method of zoning will actually work best in a particular installation.

Fortunately, those circulators are more versatile than older pumps and can be adjusted to operate at lower flow rates with the proper settings. You should probably operate them in the energy-saving proportional pressure mode (green light) most of the time.

If the pumps are currently operating in the constant-pressure or normal mode (amber light), that could be contributing to the noise problem you are experiencing.

Before redesigning the installation, try adjusting the circulator settings and see if the system operates more quietly at the lower flow rate. If that does not help then a near boiler piping redesign might be your only option.

High pitch noise is normally the signs of a high velocity flow thru a small opening.

Cavitation sounds more like marbles in the impeller due to the amount of bubbles being formed in the volute.

Use along screw driver to try and pinpoint the noise. Put one end of the screw drive on the offending area and the other on your ear. Your using it like a stethoscope. Move the screw drivwer around in a attempt to pinpoint the true area of the noise.

@steve617 Do you have a stethoscope? If not, how about a long screw driver?

Put the handle of the screw driver to your ear and then to the circulator and any other moving part to hear if the noise is more isolated. Sometimes, but not always, there is a resinating sound that came come from a vibration, from a hanger or other support that can make a soprano pitched hum as mentioned above by @EdTheHeaterMan .

Give this a try before going to all of the piping changes that might be needed. I have a small suspicion that hangers supporting the pipes and circulators could be a part of this. Start with the threaded rod that is attached to the floor below the circulators and work your way up from there.